Methods for detecting the start of a dive using various physical principles are known from the prior art.
EP Patent No. 0 689 109 granted in the name of CITIZEN WATCH CO. LTD on 16 Dec. 1998, discloses a method of this type and a portable electronic device for implementing the same. In particular, this device is provided with specific means arranged for detecting an entry into contact of the device with water, and a pressure sensor, arranged for measuring the ambient pressure value. According to this patent, the pressure sensor is powered, in a first operating mode, so as to carry out an atmospheric pressure measurement approximately once an hour in order to store the value thereby obtained as the reference pressure value. Moreover, the specific water detection means, which can for example take the form of ohmic contacts arranged on the case of the device, are permanently or periodically powered.
The ohmic contacts thus fulfil a main switch function for the circuits dedicated to the operating mode relating to diving, particularly for the pressure sensor. In fact, when the presence of water is detected at the ohmic contacts, the powering frequency of the pressure sensor is modified such that the ambient pressure measurements are carried out with a period of the order of a second, in an operating mode called the preparation mode. These measurements allow the pressure variation value between the last measured value and the last stored reference pressure value to be calculated, the variation value then being compared to a predefined pressure value constituting a dive mode trigger threshold. When the pressure variation passes the trigger threshold, the dive mode is activated. In the opposite case, the sensor it still powered for several minutes in order to monitor the evolution of the ambient pressure. Once this time period has passed, the preparation mode is deactivated and the pressure sensor is again powered with a period of the order of an hour.
Such a detection method for example enables a distinction to be made between a situation in which the person wearing the device wets the latter by washing his hands and a situation actually corresponding to the start of a dive. In this latter case, the pressure measurements carried out by the pressure sensor enable the start of a dive to be validated after activation of the preparation mode, insofar as the device first experiences contact with water before its surrounding or ambient pressure increases.
However, this type of device has a significant drawback from the point of view of its construction, residing in the need to provide specific means for detecting the presence of water around its case. In the aforecited case of thus use of ohmic contacts, it is in fact imperative to provide specific means for guaranteeing the water resistance of the case of the device in the region of such contacts, which may involve significant consequences as regards the manufacturing cost of the device. Consequently, the method described hereinbefore has a similar drawback because it is based on the implementation of specific means for detecting the presence of water.
Other methods and devices are known from the prior art which do not implement such specific means for detecting the presence of water and which exploit ambient pressure measurements to detect the start of a dive.
In particular, such devices are known in which a pressure sensor is periodically powered to measure the ambient pressure value, the results of these measurements being stored. These devices are arranged such that every time there is a new ambient pressure measurement, the variation value between this last measurement and the preceding one is calculated and compared to a value defining a trigger threshold. Once the trigger threshold has been passed, the dive mode is activated, the penultimate measured pressure value being typically stored as the reference pressure, i.e. it is supposed to correspond to the surface pressure of the body of water in which the dive is being carried out.
This type of device has, however, a drawback because the precision of the detection of the start of a dive is entirely based on the value retained for the trigger threshold.
Thus, if the value retained for this threshold is too low, the device is exposed to a risk of the dive mode being inadvertently triggered. By way of example, if the person wearing such a device descends a mountain road at a sustained pace, the device is capable of mistaking the corresponding increase in pressure for entry into water. On the other hand, if the value retained for the threshold is too high, precision as to the trigger is liable to be poor if the wearer remains in the water in proximity to the surface for some time before the start of a dive. In such a case, it may also happen that the stored reference pressure value is not correct since it was measured in the water, at a lower depth than that corresponding to the trigger threshold. Such an error can, depending upon its amplitude, have dangerous consequences for the health of the person wearing the device, particularly from the point of view of data relating to any decompression stop onto which the error would be carried over.